I’m starting a new series called “Exploring HEC-RAS”. Each post will discuss a feature in the menu items, starting with the Tools menu item in the Geometry Schematic. Today’s topic is “XS Interpolation Within a Reach”. There are two options for interpolating cross sections in HEC-RAS: “Within a Reach…”, let’s call this Option A, and […]
One of the most frustrating aspects of unsteady HEC-RAS modeling can be the model stabilization process. You know, you’ve gone to great lengths collecting the best survey/topo data and solid hydrology. Then you’ve painstakingly spent hours…possibly days entered all of that data only to find that once you press the “Compute” button, the model crashes. […]
A very convenient way to troubleshoot instability problems with very complex models is the use of a hotstart run with a stepdown scheme. Creating a stepdown scheme hotstart plan is covered in detail here. In a previous post, I explained some problems with running the stepdown scheme when you have bridges. Here I want to […]
So many of us know that if you experience instabilities at or near your inline structure, you should bump up the Inline Structure Flow Stability Factor from the default of 1 to the most stable value of 3. But what is really going on when you do this? Here’s an example of a RAS model […]
Adding to the previous topic on the Theta Implicit Weighting Factor (Theta), this post takes an objective look at how the unsteady-flow option affects model output. Theta is a weighting factor for the spatial derivative used in solving the finite difference forms of the St. Venant equations. Adjusting Theta can improve model stability or increase […]
I’ve mentioned this a lot in this blog, but I’m finding more and more that a good, solid definition of your HTab parameters up front will go a long way in helping construct a nice stable unsteady flow model in HEC-RAS. First, let me recap how they work. In unsteady flow, HEC-RAS will convert the […]
While running unsteady flow simulations in HEC-RAS instabilities may occur when transitioning from the automatically created initial condition file to the first computed time step. These instabilities can be caused by mixed flow conditions, flow splits, or poorly defined initial conditions. A hotstart is another option available for defining initial conditions for the project model. […]
Storage Areas in unsteady RAS are notoriously stable. That’s why we like to use them. Get the water out of the 1-d St. Venant unstable environment, into the 0-d stable continuity environment. However, I recently discovered a problem with storage areas that could cause your model to go unstable, or at least chug along slowly […]
Written by Chris Goodell, P.E., D. WRE | WEST ConsultantsCopyright © RASModel.com. 2010. All rights reserved.Some questions and answers related to dam breach modeling in HEC-RAS… Question. The Sunny Day model has a consistent water surface elevation from the very start of the model – it only decreases once the breach occurs. How is HEC-RAS […]